Belgian authorities have become increasingly concerned about a growing number of young men leaving to fight in Syria against President Bashar al-Assad government.
Tarik Taketloune, 20, was shot in the head and killed on Friday in a house near Aleppo, the Flemish newspaper Het Nieuwsbald said.
Taketloune's older brother Elias, 23, was arrested on his return to Belgium on June 3 and has been detained on suspicion of terrorist activities.
Taketloune is the third Belgian killed while cooperating with militant groups fighting in Syria, while there are unconfirmed reports of a fourth death, Alalam reported.
Separately the leader of the Belgian terrorists in Syria, named as Elouassaki, said in a conversation recorded by police that he had slit the throat of a Syrian soldier, according to the reports.
He may also be among a group of men speaking in Flemish and French who feature in a recent video of a man being decapitated with a knife, the newspapers said.
Europe has seen a rise in the number of its citizens entering Syria.
On April 24, EU's anti-terror chief Gilles de Kerchove told the British media that some 500 Europeans were in Syria to fight against the government of President Bashar al-Assad.
Kerchove said Britain, Ireland and France were among the EU countries to have the highest numbers of militants in Syria.